# How Do Black Holes Swallow Matter?

Rev. Green | 12:52 Fri 04th Jan 2019 | Science
How do Black Holes swallow matter? They have high gravity, but almost every object that approaches them should go into orbit in the same way that almost everything in the Solar System orbits the Sun. Even a missile fired from Earth directly towards the Sun will continue to orbit. Is the angular momentum of an object near a Black Hole destroyed by tidal effects?

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I was going to give a detailed answer but it maybe takes too long to prepare. The gist of it, though, is that you also have to consider the energy of the object in orbit, as well as its angular momentum. Objects that come near a black hole tend to have very low energies, and, as a result, don't form closed orbits, instead wanting to come close to the black hole and then...
18:11 Fri 04th Jan 2019
It is indeed possible to orbit a Black Hole safely -- if the Sun collapsed into a Black Hole we would continue orbiting it. However, most objects that come near Black Holes do not enter these stable orbits, and instead their orbits will gradually decay until at some point they are close enough to start being ripped apart.

I could also add that the final possible orbit about a Black Hole -- something like 1.5 times its radius -- is also unstable, so that the slightest kick will stop the object from safely orbiting and again force it to collapse.
If you can get your head around this, you m i g h t understand a little more. Me.....I’m spinning into infinity.
https://arxiv.org/abs/1103.6140
Not how I understand it. An object's approach has to be right to go into orbit. Try dropping an object from a great height, it doesn't immediately zoom horizontally and orbit, it falls towards the centre of the Earth. So the orbit idea doesn't really work.

As for swallowing, not really the right analogy. Stuff simply gets crushed by gravity into a infinitely dense and small point as it nears the centre (unless the physics changes from our present understanding and prevents the singularity somehow).
I'm not sure how scientifically accurate it is, but the film 'interstellar' is the best black hole example i've ever seen. The astro scientist in it also talks a lot about them, and how even planets can orbit a black hole for a fair amount of time before they're consumed.

however, it is a film, good watch though if you have any interest in space / time
OG, you’re using a scenario where the weightless vacuum of space isn’t a factor. There is already an ‘inertia’ on a dropped object and zero kinetic energy.
I think I should also add that OG has a point, and you are exaggerating the ease of reaching stable orbit. It's a sort of selection bias. By definition, we only see the objects that do still have stable orbits, because if their orbits were unstable then they would have crashed into the Sun a long time ago.

As a separate example, it's well-known that many objects that go near to Jupiter end up hitting it. Shoemaker-Levy was one spectacular example.
Surely makes little odds. Something heading for the black hole ain't going to change direction and orbit unless another force acts on it.
interesting article ZM, I remember reading a book that had some Fascinating ideas on black holes. For example a black hole of sufficient size could have an event horizon, within the outer layers of which the density can be no more that that we live in now. For most people "black hole" means event horizon of the singularity and in this sense the properties of such still allow for potential for planets life stars etc all with an event horizon. Eg we could all be in one now. Fanciful postulation perhaps but the maths does allow it.
sorry Rev, to answer the original question, jim is correct in the first answer, most things do not attain a stable orbit.
Tora, May be black holes are required to even out gravity in the universe. You may well be right in that we (that could be a singular ‘we’ a ‘Earth’ or a collective ‘we’ if there are other inhabited planets) may all live within the influence of BHs. This would mean that it’s definitely possible to live within their influence, answering the last sentence of Rev’s postulation.

Jim - is the universe too big for BHs to work as combined ‘gravity anchors’, allowing the rest of ‘it’ to hang together?
Black Holes aren't even close to being the dominant provider of mass or gravity in the Universe, so in themselves can do little or nothing to hold the Universe together.

Damn.
Question Author
Sorry, folks. I'm no clearer. Old Geezer says " Try dropping an object from a great height, it doesn't immediately zoom horizontally and orbit, it falls towards the centre of the Earth. So the orbit idea doesn't really work." That's fine at the poles, but elsewhere the object has angular momentum which it needs to lose to head to the centre of the Earth.
My question is, as most objects have angular momentum that will keep them in orbit, how do they lose it?
I was going to give a detailed answer but it maybe takes too long to prepare. The gist of it, though, is that you also have to consider the energy of the object in orbit, as well as its angular momentum. Objects that come near a black hole tend to have very low energies, and, as a result, don't form closed orbits, instead wanting to come close to the black hole and then fly away again.

Most objects that go near an large body then have one of two things happen to them: they either miss, and are able to fly off, or they were heading close enough to the central mass that they'll get captured; or they hit it head on and are destroyed.

The point of a black hole is that it's quite easy to get very close indeed to the central object, and, as a result, get captured -- but then, because the object is super close, it is too close to form a stable orbit, and its angular momentum is simply not enough to keep it away from the black hole.

The maths for this is fairly well-understood, but for various reasons I don't want to go into it here. Suffice it to say that your intuition makes sense when you are millions of miles away from a black hole, but not so much when you are only a few hundred miles away.
// sorry Rev, to answer the original question, jim is correct in the first answer,//

golly Jimbo - this is like a AB gold star for your fridge !
// but for various reasons I don't want to go into it here. //

oh I can: AB is replete wiv thickos who make one line quips like : "hawking radiation wot dat den?"
I threw the following together before I saw Jim's response., but here goes:

Angular momentum is conserved - even within a black hole, just as mass is conserved within a black hole. It's one of those properties that tends to remain constant in a universe outside unusual environments.

Black holes, of course are fairly unusual environments, so the physics of a rotating black hole are a lot more complex than those of a stationary one.

Nevertheless, the angular momentum of a black hole is still (kind of) the sum of the angular momenta of the contributing masses.

But, it depends a bit on your frame of reference.

A body falling onto the BH might be spinning on its axis, and so has some angular momentum, but it might also be orbiting the BH in one specific plane, so it has some angular momentum from that motion as well. The amount of angular momentum added the BH when the object falls in depends on the frame of reference chosen to measure the angular momentum.

In a classical case, and assuming the body is a long distance from the singularity, so that the forces on one side of the body are not significantly different from the forces on the other side, the body falls toward the singularity.

It is rare for the trajectory of the falling mass to be directed perfectly at the singularity, so the body falls in the general direction of the black hole with some momentum along a particular trajectory.

Even if the falling body is falling directly toward the singularity, there is likely to be some extraneous matter orbiting the BH, that will deflect the body away from its targetted trajectory.

As the gravitational forces from the BH act on the body, they accelerate it in a direction directly in line with the singularity. This gravitational force transfers some Angular Momentum from the BH to the falling body (just as a man-made satellite can slingshot around Jupiter gaining angular momentum from the interaction with Jupiter's gravitational field).

Depending on the mass of the BH, the orbital radius of the falling body becomes closer and closer to the BH. In order to conserve angular momentum, the orbital speed accelerates. Depending on the mass of the BH, the forces involved in this tight orbit might tear the falling body apart through mechanical forces.

If the BH has a very large mass, the gravitational gradient at the event horizon is relatively shallow, and the falling body moves into an unstable decaying orbit around the BH. Eventually, the falling body moves beyond the event horizon, and we can know no more about its trajectory, though we estimate that it continues in the decaying orbit, until tidal forces rip it apart, and the angular momentum is transferred back to the BH.

At least, that's how I think it works. Happy to be contradicted.
Wow! And I'm struggling with David Copperfield!
Who's winning ?

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